This application claims the benefit of Japanese patent application number 2000-054496, filed Feb. 29, 2000, the contents of which are incorporated herein by reference.
1. Field of Invention
The present invention relates to a fluorescent lamp having a light-transmitting glass tube, which does not substantially contain lead (Pb).
2. Description of Related Art
To save our environment, demand has increased for a fluorescent lamp having a glass tube which substantially does not contain lead (Pb). The glass tube is made of a soda-lime glass, for example, having sodium oxide (Na2O) of 15 to 17 weight %, and a softening temperature of about 690 degrees centigrade. A circular fluorescent lamp having the soda-lime glass tube may be manufactured, for example, as follows:
(1) coating a luminescent compound solution on the inner surface of a straight glass tube,
(2) baking the coated layer on the straight glass tube at a predetermined temperature to form a phosphor layer,
(3) sealing each end of the glass tube with a stem having an electrode,
(4) forming the straight glass tube into a circular glass tube by bending the straight glass around a drum while heating the straight glass tube at a softening temperature of the glass or more,
(5) exhausting the circular glass tube via an exhaust tube and introducing rare gas and mercury (Hg) into the circular glass tube, and
(6) sealing the exhaust tube.
In this case, the efficiency of the phosphor layer is reduced as a result of the heat of the above-mentioned forming process (4), so that the total luminous flux tends to drop, since the chemical characteristics of the phosphor are changeable at the temperature of the forming process (4).
The mercury (Hg) in the glass tube slightly migrates into the glass tube during the lamp operation, so that the sodium (Na) of the glass tube reacts with the mercury (Hg). As a result, the glass tube slightly blackens, so that the luminous flux decreases.
The sodium (Na) occasionally migrates to the inner surface of the glass tube because of the heat of the forming process, so that sodium (Na) reacts with mercury (Hg) and makes a mercury compound on the inner surface of the glass tube. As a result, the mercury compound reduces the luminous flux, because the mercury compound absorbs visible light generated by the discharge of the fluorescent lamp. Furthermore, the sodium (Na), migrating to the inner surface of the glass tube, migrates into the phosphor layer, and causes the phosphor to become slightly brown. It is known that the amount of coloring caused by sodium (Na) is proportional to the amount of heat supplied to the straight glass tube in order to form it. As the heat is increased, sodium (Na) migrates more to the inner surface of the glass tube and migrates into the phosphor layer more.
Also, when mercury (Hg) reacts with sodium (Na), the quantity of mercury (Hg) in the glass tube decreases. As a result, the fluorescent lamp occasionally can not maintain stable discharge because of a lack of mercury (Hg). Therefore, generally, such fluorescent lamp contains an excessive quantity of mercury (Hg), e.g., a total amount of about 10 mg or more, or about 0.02 to about 0.03 mg/cm3 or more.
Furthermore, Japanese Laid Open Patent Application HEI 10-324540 discloses a glass composition not containing lead (Pb), for use in a stem. A part of a pair of conductive wires are embedded in the stem. Such glass composition is used mainly because the thermal expansion coefficient of the glass is similar to that of the conductive wires. However, such stem glass composition is not used for the glass tube. That is, it is not disclosed that the glass composition should be selected so that it is easily formed into a predetermined shaped tube, e.g., a circular tube or U-shaped tube, and so that reduction of luminous flux is reduced during manufacturing or over the life of the lamp. Moreover, it is not disclosed that the sodium (Na), which migrates to the inner surface of the glass tube by the heat, reacts with mercury (Hg), or migrates to the phosphor layer.
According to one aspect of the invention, a fluorescent lamp comprises a light-transmitting glass tube having a softening temperature of about 685 degrees centigrade or less. The light-transmitting glass tube does not substantially include lead (Pb), and does include: about 1 to about 11 weight % sodium oxide (Na2O), about 1 to about 10 weight % potassium oxide (K2O), 0 to about 3 weight % lithium oxide (Li2O), wherein the total amount of sodium oxide (Na2O), potassium oxide (K2O), and lithium oxide (Li2O) is about 5 to about 20 weight %, and about 0.1 to about 0.5 weight % antimony oxide (Sb2O3). A stem, having an electrode, is arranged at each end of the light-transmitting glass tube.
According to another aspect of the invention, a method for manufacturing a fluorescent lamp comprises:
coating a luminescent compound on the inner surface of a straight glass tube;
wherein the straight glass tube has a softening temperature of about 685 degrees centigrade or less, does not substantially include lead (Pb), and includes:
sodium oxide (Na2O) about 1 to about 11 weight %,
potassium oxide (K2O) about 1 to about 10 weight %,
lithium oxide (Li2O) 0 to about 3 weight %, wherein the total amount of sodium oxide (Na2O), potassium oxide (K2O), and lithium oxide (Li2O) is about 5 to about 20 weight %, and
antimony oxide (Sb2O3) about 0.1 to about 0.5 weight %;
baking the coated layer on the straight glass tube at a predetermined temperature to form a phosphor layer;
sealing each end of the glass tube with a stem having an electrode;
bending the straight glass while heating the straight glass tube at the softening temperature of the glass or more;
exhausting the glass tube via an exhaust tube and introducing rare gas and mercury (Hg) into the glass tube; and
sealing the exhaust tube.
According to another aspect of the invention, a lighting fixture comprises the fluorescent lamp. A body holds the fluorescent lamp. A ballast is arranged on the body for supplying the electric power to the fluorescent lamp.
These and other aspects of the invention are further described in the following drawings and detailed description of the invention.